The conversion of abundant and renewable C1 source and carbon dioxide (CO2) to commodity chemicals has received a lot of recent attention in part as a sustainable solution for carbon dioxide recycling and reduction. Particularly, reaction of carbon dioxide and epoxides to form polycarbonates or cyclic carbonate is considered as one of the promising industrial processes.
The cyclic carbonate is used as raw materials of polycarbonate, intermediates for cosmetics and pharmaceutical process, oxyalkylation agents in a dye synthesis process, protective agents of a process equipment, and solvents of a fiber production process. In addition, the cyclic carbonate may be used as an intermediate in preparing alkylene glycol from alkylene oxide. Recently, a range at which the cyclic carbonate is used has been continuously expanded to a solvent of a polymer electrolyte of a secondary battery, etc.
Several catalytic systems such as transition metal complexes or organocatalysts were reported to promote the reaction of carbon dioxide and alkylene oxide. As a well-known example, there are salen-based complexes (salen=N,N′-bis(salicylidiene)ethylenediamine) of Al, Cr, Mn, Co and Zn that are recognized to be effective for both polycarbonate and cyclic carbonate synthesis. [Cr(salen)] and [Co(salen)] complexes for polycarbonate synthesis were developed; meanwhile, binuclear [Al(salen)]2O complex for selective cyclic carbonate synthesis was developed.
In addition to metal-dependent selectivity, research into ligand modification in a complex to improve reactivity and selectivity of a catalyst was conducted, and a highly active carbon dioxide/propylene oxide copolymerization catalyst prepared by incorporating co-catalyst ammonium salts in [Co(salen)] complexes has been reported, which is being developed for industrial production of polypropylene carbonate.
In addition, it has been reported that mononuclear Al complexes and dinuclear Fe complexes based on amino tris(phenolate) ligands have excellent reactivity and selectivity in the cyclic carbonate synthesis.
As other different methods, for example, Japanese Patent Laid-Open Publication No. S56-128778 discloses cyclic carbonate prepared in a catalyst system consisting of an alkali metal compound and a crown compound, Japanese Patent Laid-Open Publication No. S59-13776 used quaternary ammonium and alcohol, WO 2005/003113 used tetraalkyl phosphonium halide, and Korean Patent Laid-Open Publication No. 2005-0115694 used a catalyst system consisting of a metal salt and an aliphatic cyclic amine salt. However, these synthesis methods have problems in that since a reaction yield is low under mild reaction conditions, high temperature and a long period of reaction time are required to increase yield, and moisture contents of raw materials, i.e., carbon dioxide and alkylene oxide should be controlled to be hundreds of ppm or less.
U.S. Pat. No. 5,283,356 discloses a phthalocyanine catalyst including Co, Cr, Fe, Mn, Ni, Ti, V, Zr, etc., and Japanese Patent Laid-Open Publication No. H7-206547 discloses a catalyst substituted with rubidium (Rb) or cesium (Cs) ions instead of hydrogen ions of hetero polyacid, respectively. However, these catalysts are high cost, wherein a reaction temperature is high as 120° C. to 180° C., and a yield is low as 30 to 90%.
As described above, the catalyst used for industrial synthesis of the cyclic carbonate according to the related art has problems in that reaction conditions are complicated, for example, a reaction temperature should be high, a moisture content of raw materials should be significantly low, etc., and selectivity and yield are low, and a reaction time is long.